1. Field Of The Invention
This invention relates to the molding of cellular plastic products such as polyurethane foam products and more specifically is directed to a novel and useful method for bursting the cells of the foam to avoid shrinkage upon demolding and/or to produce desirable properties including high resiliency in polyurethane foams.
2. Description Of The Prior Art
In the production of molded high resilience polyurethane foam products such as automotive seating, it is necessary to crush or otherwise open the cells of the foam as quickly as possible after molding the product in order to avoid shrinkage. If the cells are not opened, cooling and contraction of gases within the closed cells provides a pressure drop within the closed cells which is sufficient to allow atmospheric pressure acting on the exterior of the cell walls to at least partially collapse the cells to result in undesirable shrinking of the molded product. Opening of the cells also provides a desirable increase in resiliency which is of considerable value in producing cushiony articles such as automotive seating.
A great deal of effort has been expended to devise ways and means for opening the cells of molded polyurethane foam products and several techniques are in current use. In each of the techniques currently employed, the molded foam product is removed from the mold and quickly subjected to the cell opening technique before the gases within the cell have had an opportunity to contract to the point of cell collapse and shrinkage.
One technique being used is to remove the foam product from the mold and quickly spear it with a single-prong or multi-prong air tube and inject air at various locations within the foam product. The injected air is intended to maintain the foam structure and thus avoid shrinkage. Air injection is of limited effectiveness but is either labor-intensive or requires special machinery for injecting the air. It also frequently results in the tearing of the molded foam and in all cases punctures the outer skin of the product.
A much used practice involves removing the molded foam article from the mold and quickly subjecting it to roller crushing. While roller crushing is quite effective in improving the resiliency of the foam article, it is limited to use on articles that contain no three-dimensional reinforcing or support members, for example, automotive seating that contains molded-in steel frames. Vacuum crushing has also been employed with some success but requires special vacuum equipment and high energy costs to operate. As with roller crushing, vacuum crushing requires special handling in quickly removing the foam article from the mold, quickly placing it in a vacuum chamber, applying vacuum and then removing it from the chamber. Roller crushing requires extra rolling equipment and added energy costs to operate. All heretofore known systems also require the consumption of time to provide the bursting of cells.
All mechanical crushing or cell opening techniques employ the application of either an external or an internal force to cause the internal cell pressure to exceed that which the cell windows can contain resulting in window rupture and an effective open cell structure. More specifically, the pressure differential of a specific cell and its environment, i.e., nearest neighbors, is responsible for cell window blowout. The known and commonly used mechanical cell opening techniques of roller crushing, vacuum crushing or air injection all function in this manner and in all cases the technique is employed after demold of the high resilience part.
U.S. Pat. No. 4,285,893 discloses the use of vacuum to burst the cells of a cellular molded product while it is still in the mold to avoid the need for handling the molded product, i.e., removing it from the mold and transferring it to a vacuum chamber for bursting cells. However, the process of the patent requires special vacuum equipment, specially constructed molds and increased energy consumption for operating the vacuum system. Most significantly, the process of the patent retains the molded product within the mold to complete polymerization. i.e., 2 to 5 minutes after the completion of foaming before applying vacuum and thus completely misses the critical time window utilized by this invention for bursting the cells while the product is still in the mold. The process of the patent merely involves the carrying out of the usual well-known vacuum cell bursting technique but applies the vacuum to the molded product while it is still in the closed mold.
No prior art is currently known teaching the utilization of the critical time window discovered by this invention for bursting cells by exposing the molded product to atmospheric pressure at a time when the polyurethane has cured sufficiently to avoid collapse of the cells but has not cured and cooled to the extent that the pressure of gases contained within the cells is not sufficient to burst the cell walls upon exposure to atmospheric pressure.